This invention relates to a method of processing a color video signal using a memory.
There have been known techniques of still image process, mosaic process, noise reduction process, etc. for video signals using memories. An example is described, for example, in Nikkei Electronics, No. 406 (Oct. 20, 1986), pp. 195-214, in which a VTR incorporates a field memory and it is used to improve the picture quality in still image playback mode, slow playback mode and search mode.
When the still image playback command is encountered during the operation of the magnetic tape at the normal tape feed speed, a video signal for one field is extracted and written in the field memory, and a still image is reproduced by reading out the 1-field video signal from the field memory iteratively. In the case of slow playback, the magnetic tape is fed intermittently in a speed range of 1/5 to 1/30 time the normal playback speed, and video signals for one field is saved in the field memory during the period when the magnetic tape is stopping and the video signal is read out of the field memory during the period when the magnetic tape is moving. This technique is intended to reproduce a still image instantaneously and a smooth slow motion image regardless of the tape feed speed. In the case of search playback mode, signal portions with satisfactory tracking are extracted from signals that are picked up in the period when the magnetic head scans the tape twice and stored in the field memory, and these portions are connected to reproduce a video signal for one field. As a result, a playback image, with noise bars being removed, is reproduced. The field memory is written and read out independently, and a new sync signal is appended to the readout video signal thereby to eliminate the skew distortion.
In order for this conventional technique to prevent the discontinuity in phase of the color subcarrier at the connection of signals, the signals stored in the field memory consist of the luminance signal and color difference signals separated from the color video signal, i.e., component signals. Such a scheme of process is called "component signal processing method", and the adoption of this processing method also yields the stroboscopic effect, mosaic effect and solarization effect.
Another example is described, for example, in NEC Technical Journal, Vol. 40, No. 3 (March 1987), pp. 49-52, in which a field memory is used for the special effect and a 1-field delay element of cyclic noise reducer is used to reduce noises in normal playback mode.
A further scheme for providing a picture-in-picture effect is known (e.g., described in Nikkei Electronics, No. 406 (Oct. 20, 1986), pp. 178-179).
For the process of the special playback and noise reduction using a field memory, there have been a case of composite signal processing in which the color video signal is processed directly and a case of component processing. The composite signal processing requires a smaller memory capacity, but it is necessary for this method to device the retention of continuity of the color subcarrier before and after the signal processing. Therefore, the circuit becomes complex, and another problem is the degradation of picture quality, particularly the fidelity of color.
In contrast, in component signal processing, the signals stored in the field memory are the luminance signal and color difference signals in the base band. The sync signal and color burst signal need not be stored in the field memory, but instead they can be appended to the signal read out of the field memory, and therefore it does not need to device the continuity of the color subcarrier.
As a recent effort of enhancing the resolution of reproduced image, the luminance signal has its frequency band expanded to include the frequency band of the chrominance signal. However, the introduction of a color video signal including a broad-band luminance signal to the component signal processing creates a color flicker, resulting in a degraded picture quality. The following describes the color flicker by taking an example of still image reproduction.
In the still image processing based on the component signal processing, a color video signal for one field is separated into a luminance signal and a chrominance signal, and these signals are stored in a field memory. In the case of the NTSC standard television system, in which 525 scanning lines are produced by 1-interlace scanning, one field has 262.5 lines to be stored in the field memory. For the still image processing, each component signal stored in the field memory is read out iteratively. With the intention of avoiding a line flicker caused by interlace scanning, it is designed to read out 262 scanning lines or 263 scanning lines alternately for every field from the field memory.
The luminance signal read out of the field memory is appended with a sync signal and blanking signal. The color subcarrier is modulated with the readout color difference signals, and it is appended with a color burst and other signals to form a chrominance signal. The luminance signal has its vertical sync signal position set so that fields have 262 or 263 scanning lines alternately and the lines are laid by non-interlace scanning.
In the case of expanding the frequency band of the luminance signal to the extent of including the frequency band of the chrominance signal with the intention of enhancing the resolution of a reproduced image, a comb line filter is used as a separation circuit for separating the color video signal into the luminance signal and chrominance signal. However, due to an adjustment error of the comb line filter, the separated luminance signal includes a residual of chrominance signal in general. In addition, due to the crosstalk in the circuit or wiring following the comb line filter, the chrominance signal can leak into the separated luminance signal. Such a residual chrominance signal in the luminance signal will be termed crosstalk signal component .DELTA.C.
FIGS. 1A-1D are diagrams explaining the following of the chrominance signal read out of the field memory for each field, in the case of reproducing a single still image signal. In the figure, having a horizontal time axis and a vertical axis of signal level, signal C is the chrominance signal component in the composite color video signal formed by merging the luminance signal and chrominance signal, signal C' is the chrominance signal component immediately before it is stored in the memory, and .DELTA.C is the residual chrominance signal component in the luminance signal as mentioned previously. Because of still image reproduction, the component signals (luminance signal and chrominance signal) for one field (263 horizontal scanning lines) is stored in the field memory. For playback, the same component signals are read out iteratively for each field from the field memory. FIG. 1A shows the field signal which is read out first. FIG. 1B shows the field signal of the second readout. Although the first and second field signals are the same field signal read out from the same field memory, the first and second fields are displaced from each other by one scanning line for the purpose of interlacing. The first and second fields in combination form a video signal for one frame. Similarly, the third and fourth fields, and thereafter an odd-numbered field and adjoining even-numbered field form a respective frame.
The luminance signal including the crosstalk chrominance signal component .DELTA.C for one field is stored in the field memory, which is then read out such that configuous fields have 262 or 263 scanning lines alternately. In the following, attention is paid on the first through fourth fields. Since the component signals of the same field are read out iteratively from the field memory, the crosstalk chrominance signal component .DELTA.C has the same phase relation with the horizontal sync signal (not shown, it is assumed to be located at the left end of each scanning line) among all fields, as shown in FIGS. 1A-1D.
In the NTSC system, the color subcarrier frequency f.sub.sc and the horizontal sync frequency f.sub.H relates with each other as follows. ##EQU1## Accordingly, each scanning line has a period which is an odd multiple of half the color subcarrier period.
The color difference signals read out of the field memory modulate a continuous color subcarrier to form a chrominance signal C'. The chrominance signal C' is shown by the dash-dot line in the FIGS. 1A-1D. It is assumed that the crosstalk chrominance signal component .DELTA.C and chrominance signal C' have the same phase in the first field. The color subcarrier frequency f.sub.sc horizontal sync frequency f.sub.H are in the relation of equation (1), the crosstalks chrominance signal component .DELTA.C and chrominance signal C' have their phase reversing with respect to the horizontal sync signal for every scanning line, the first field (odd-numbered field) comprises an odd number of scanning line, and a signal of the same field is read out iteratively from the field memory in the still image process. Based on these conditions, the crosstalk chrominance signal component .DELTA.C and chrominance signal C' have opposite phase from each other. Similarly, the second field (even-numbered field) comprises an even number of scanning lines, and therefore in the third field, the crosstalk chrominance signal component .DELTA.C and chrominance signal C' have opposite phases from each other as in the second field. The third field comprises an odd number of scanning lines, and therefore in the fourth field, the crosstalk chrominance signal component .DELTA.C and chrominance signal C' have the same phase. The change in the phase relation between the crosstalk chrominance signal component .DELTA.C and chrominance signal C' in every field is repeated for every fourth field.
When the produced chrominance signal C' and the luminance signal are merged to form a composite color video signal, the resulting composite color video signal has its chrominance signal C composed of the chrominance signal C' and crosstalk chrominance signal .DELTA.C. The chrominance signal C is the chrominance signal C' added by the crosstalk chrominance signal .DELTA.C in the first and fourth fields and it is the chrominance signal C' subtracted by the crosstalk chrominance signal .DELTA.C (addition in opposite phases) in the second and third fields, as shown by the solid line in FIGS. 1A-1D. As a result, the amplitude of chrominance signal C increases in the first and fourth fields and it decreases in the second and third fields.
On this account, the chrominance signal C has its amplitude increasing or decreasing at an interval of four fields, resulting in an increase or decrease of color saturation at an interval of four fields on the screen, which creates a color flicker.
Although the above explanation has assumed the crosstalk chrominance signal .DELTA.C and chrominance signal C' to have the same phase, the color subcarrier of the chrominance signal C' is not necessarily always in-phase with the crosstalk chrominance signal C'. Therefore the phase relation in the first and fourth fields differs from that in the second and third fields. Namely, the chrominance signal C has different phase relations with the horizontal sync signal between these field pairs, resulting in the occurrence of color flicker in the hue direction. Although the above-mentioned color flicker does not arise in the color video signal processing system which deals with the luminance signal as a narrow-band signal, it becomes a serious problem when a broad-band luminance signal is adopted for the enhancement of screen resolution.